The present invention relates generally to articles such as sheeting having distinctive reflective properties. The invention has particular application to polymeric mirrors, polarizers, and retroreflectors.
The term "retroreflective" as used herein refers to the attribute of reflecting an obliquely incident light ray in a direction antiparallel to its incident direction, or nearly so, such that it returns to the light source or the immediate vicinity thereof. The term "dichroic" refers to the separation of light into at least two spectral components by selectively reflecting light in a first wavelength range and selectively transmitting light in a second wavelength range. The term "mirror" as used herein refers to an extended body that specularly reflects on-axis linearly polarized light substantially the same regardless of the orientation of the linear polarization with respect to an axis in the plane of the body. The term "polarizer" as used herein refers to an extended body that transmits on-axis linearly polarized light substantially differently depending on the orientation of the linear polarization with respect to an axis in the plane of the body. Other terms are defined in the glossary at the end of the specification.
PCT Publication No. WO 95/17692 (Ouderkirk et al.), incorporated herein by reference, discloses multilayer polymeric films configured as reflective polarizers in combination with various structured surface materials which confine light to a relatively narrow viewing zone. Multilayer films having birefringent layers are discussed, including films where the out-of-plane refractive indices of adjacent layers are matched. Structured surface materials include those having a plurality of triangular prisms as well as symmetric cube corner sheeting. A discussion of the multilayer films configured as mirrors is also included, including an infrared reflecting mirror with on-axis transmission of over 80% from 400 to 700 nm with a dip in transmission (increased reflectance) for some wavelengths greater than 700 nm.
U.S. Pat. No. 4,175,775 (Kruegle) discloses a photo I.D. card with a photographic image hidden behind a filter region and placed in front of a high gain retroreflective material. The filter region absorbs virtually all of the light impinging on it, and may comprise a half-silvered mirror but "advantageously comprises" a filter material which transmits infrared or ultraviolet light but blocks substantially all visible light.
PCT Publication No. WO 97/11353 (Wesolowicz et al.) discloses a laser-based target detection system applicable to search and rescue operations. Specially prepared retroreflective elements are discussed in which a base retroreflective element reflects laser energy at a first wavelength but affects an optical property separate from the reflection at the first wavelength. In the case of a sheet retroreflector, a protective polymer layer is disclosed which is conducive to be doped with a dye to provide the spectrally sensitive coating. Alternative wavelength discriminating materials discussed include a dielectric stack.
Two known types of retroreflective sheeting are microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes called "beaded" sheeting, employs a multitude of microspheres typically at least partially imbedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes, vapor coats) to retroreflect incident light. Illustrative examples are disclosed in U.S. Pat. Nos. 3,190,178 (McKenzie), 4,025,159 (McGrath), and 5,066,098 (Kult). Cube corner retroreflective sheeting comprises a body portion typically having a substantially planar front surface and a structured rear surface comprising a plurality of cube corner elements. Each cube corner element comprises three approximately mutually perpendicular optical faces. Examples include U.S. Pat. Nos. 1,591,572 (Stimson), 4,588,258 (Hoopman), 4,775,219 (Appledorn et al.), 5,138,488 (Szczech), and 5,557,836 (Smith et al.). It is known to apply a seal layer to the structured surface in a regular pattern of closed polygons which form isolated, sealed cells to keep contaminants away from individual cube corners. Heat and pressure used to form the cells destroys or deforms cube corner elements located along the polygon boundaries.
Flexible cube corner sheetings such as those disclosed in U.S. Pat. No. 5,450,235 (Smith et al.) are also known and can be incorporated in embodiments described below.
There is a continuing need for articles with novel reflective characteristics which can be made in large sheets at relatively low unit cost. Such articles can be incorporated in a variety of end use applications such as authentication, conspicuity, personal safety, search and rescue, toys, games, and decorative articles.